The present invention relates to electrochemical fuel cells, and more particularly to the incorporation into a fuel cell stack of one or more specialized sensor fuel cells for detecting problematic conditions before the other cells in the stack are adversely affected by those conditions.
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d) which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit.
At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer to form a reaction product.
In fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H2xe2x86x922H++2exe2x88x92
Cathode reaction: 1/2O2+2H++2exe2x88x92xe2x86x92H2O
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to or across the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell. Separator plates typically do not have flow passages formed in the surfaces thereof, but are used in combination with an adjacent layer of material which provides access passages for the fuel and oxidant to the respective anode and cathode electrocatalyst, and provides passages for the removal of reaction products.
Two or more fuel cells can be electrically connected together in series to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field or separator plate can serve as an anode plate for one cell and the other side of the fluid flow field or separator plate can serve as the cathode plate for the adjacent cell. Such a multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates, with the series of fuel cell assemblies interposed between the pair of end plates. The stack typically includes inlet ports and manifolds for directing the fluid fuel stream (such as substantially pure hydrogen, methanol reformate or natural gas reformate, or a methanol-containing stream in a direct methanol fuel cell) and the fluid oxidant stream (such as substantially pure oxygen, oxygen-containing air or oxygen in a carrier gas such as nitrogen) to the individual fuel cell reactant flow passages. The stack also commonly includes an inlet port and manifold for directing a coolant fluid stream, typically water, to interior passages within the stack to absorb heat generated by the fuel cell during operation. The stack also generally includes exhaust manifolds and outlet ports for expelling the depleted reactant streams, and the reaction products such as water, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack. In a power generation system various fuel, oxidant and coolant conduits carry these fluid streams to and from the fuel cell stack.
Typically, fuel cell stack performance is monitored by detecting the voltage of individual cells or groups of cells in the stack. A typical stack generally comprises 30 to 200 individual cells. Voltage detection of individual cells or groups of cells is expensive and requires a complex data acquisition system and control algorithm to detect and identify a voltage condition outside a preset voltage range and to take corrective action or shut down the stack until normal operating conditions (i.e. conditions within a desired or preferable range) can be restored. A typical approach to monitoring fuel cell performance using voltage detection is described in U.S. Pat. No. 5,170,124. The ""124 patent describes an apparatus and method for measuring and comparing the voltages of groups of cells in a fuel cell stack to a reference voltage. If the measured and reference voltages differ by more than a predetermined amount, an alarm signal or process control procedures can be initiated to implement a shut-down sequence or commence remedial action. While this voltage detection approach identifies the existence of an out-of-bounds condition, the approach is imprecise as to the source and/or nature of the problem which triggered the out-of-bounds condition.
In an electrochemical fuel cell stack comprising a plurality of fuel cells, each of the fuel cells comprises an anode comprising an anode electrocatalyst, a cathode comprising a cathode electrocatalyst, and an ion exchange membrane interposed between the anode and the cathode. At least one of the plurality of fuel cells is a sensor cell which has at least one structural dissimilarity with respect to the remaining fuel cells of the plurality. During operation of the stack, the structural dissimilarity induces an electrical and/or thermal response in the sensor cell which is not simultaneously induced in the remaining fuel cells under substantially the same operating conditions.
Thus, the sensor cell preferably operates under substantially the same conditions as the remaining non-sensor cells. However, in response to a change in a particular conditions an electrical and/or thermal response (preferably a voltage change) is induced in the sensor cell which is not simultaneously induced in the remaining fuel cells. The sensor cell reacts differently to, and can therefore be used to detect, undesirable conditions before they adversely affect other cells in the stack. The different electrical and/or thermal response of the sensor cell to the particular operating condition may provide diagnostic information, or a warning signal, or may be used to initiate a specific correction sequence to restore the stack to normal operating conditions or to shut down the stack if normal conditions cannot be restored. More than one type of sensor cell, specific to different types of conditions, may be employed in a fuel cell stack.
During operation of a fuel cell stack the anodes have a fuel stream directed thereto, typically via anode flow fields, the cathodes have an oxidant stream directed thereto typically via cathode flow fields, and each of the fuel stream and the oxidant stream directed to the fuel cells of the stack have an inlet pressure and a stoichiometry associated therewith. Each of the plurality of fuel cells generate an electric current per cathode unit area, product water at the cathode and heat at a nominal operating temperature. Preferably, the structural dissimilarity induces an electrical and/or thermal response in the sensor cell which is not simultaneously induced in the remaining fuel cells under substantially the same operating conditions of reactant supply, for example, cell inlet reactant pressure and stoichiometry. Preferably, the plurality of fuel cells are manifolded to be supplied with reactants in a parallel supply configuration. This arrangement generally makes it easier to ensure that the cells are operating under substantially the same conditions with respect to reactant supply.
Sensor cells incorporated in a stack can also serve as useful power-producing cells. Thus, during operation of the stack to produce electrical power the sensor cell(s) and the remaining cells are connected to provide electrical power. A variable electrical load may be applied across the fuel cell stack comprising the sensor cell(s).
In the above embodiments, it is preferably a voltage change which is induced in the sensor cell but not simultaneously induced in the remaining fuel cells, in response to a change in a particular stack operating condition. Typically the sensor cell is designed so that its voltage will drop before, or more rapidly than, the voltage of other cells in the stack in response to an undesirable operating condition.
Other responses which may be induced in the sensor cell but not simultaneously induced in the remaining fuel cells, in response to a change in a particular stack operating condition, include changes in electric current per cathode unit area (current density) in the sensor cell; changes in the voltage distribution within a component of the sensor cell; changes in electrical resistance in the sensor cell; and/or temperature changes in the sensor cell.
The voltage (or other characteristic) of the sensor cell may or may not be substantially the same as the remaining cells during normal operating conditions, but it is the difference in, its reaction to a change in conditions which is important to its function as a sensor cell.
Various structural dissimilarities may be incorporated in the sensor cell to provide it with enhanced sensitivity to particular stack operating conditions. For example, in preferred embodiments, at least one of the anode and the cathode in the sensor cell may have an electrochemically active area less than the electrochemically active area of the remaining fuel cells. Reactant access to at least one of the anode and cathode electrocatalysts in the sensor cell may be impeded relative to in the remaining fuel cells, for example, by blocking or eliminating some of the reactant access passages or channels in the porous electrodes, or in the flow field plates, or by masking certain areas of the cell. The electrocatalayst loading in at least one of the anode and the cathode of the sensor cell may be less than the electrocatalyst loading in the remaining fuel cells. In general, a decrease in the electrochemically active area and/or a decrease in the electrocatalyst loading would increase the performance sensitivity of the sensor cell to various undesirable fuel cell operating conditions. However, such an approach alone may not be sufficient if the sensor cell is to differentiate between different conditions that affect performance.
In other preferred embodiments, the sensor cell reactant flow field configuration may differ from the reactant flow field configuration of the remaining fuel cells. For example, the sensor cell reactant flow field configuration may induce a pressure drop which is greater than the pressure drop induced in the reactant flow field configuration of the remaining fuel cells. The sensor cell may comprise an anode or cathode electrocatalyst which differs in composition from the corresponding anode or cathode electrocatalyst in the remaining fuel cells. In stacks in which each of the fuel cells has a coolant stream directed along a coolant flow field in thermal communication with the each of the fuel cells, the sensor cell coolant flow field configuration may differ from the coolant flow field configuration of the remaining fuel cells.
The structural dissimilarity is selected to render the sensor cell more sensitive to changes in one or more particular stack operating conditions, for example, fuel stoichiometry, oxidant stoichiometry, carbon monoxide poisoning of the anode electrocatalyst, air bleed level in the fuel stream, fuel cell operating temperature, methanol poisoning, flooding of an electrode, dehydration and/or mass transport issues. In a direct methanol fuel cell stack, it may be useful to incorporate a sensor cell wherein the structural dissimilarity renders the sensor cell more sensitive to methanol concentration. In some embodiments the condition is exacerbated in the sensor cell so it reacts earlier, whereas in other embodiments the sensor cell is merely more sensitive to the condition.
Thus in preferred embodiments, sensor cells are more sensitive to at least one particular type of undesirable fuel cell operating condition (compared to other cells in the stack) such as:
low reactant stoichiometry;
excessive reactant stoichiometry;
flooding;
low or excessive fuel cell operating temperature;
carbon monoxide poisoning or other types of poisoning of the electrocatalyst;
reactant dilution, such as by a build-up of inert species like nitrogen in the reactant stream;
dehydration.
Sensor cells are affected by one or more of the above, or other, undesirable conditions in advance of the other cells. They may be used to prevent such undesirable conditions from causing general stack underperformance or damage by detecting the need for, and preferably triggering, corrective action. The early detection of undesirable or potentially deleterious operating conditions is particularly important in larger stacks employing many individual fuel cells because of increasing demands for stack reliability and longer stack lifetime. The incorporation of one or more sensor cells generally reduces the cost of and simplifies the stack control system, and also improves stack safety and reliability.
A method of operating an electrochemical fuel cell system comprising a fuel cell stack and a sensor cell, as described in any of the above embodiments, comprises:
monitoring at least one of an electrical and thermal operating parameter of the sensor cell and the same parameter of at least one of the remaining cells;
comparing the monitored operating parameter of the sensor cell with that of the at least one remaining cell;
generating an output signal if the compared parameters differ by more than a predetermined threshold amount indicative of an undesirable operating condition.
Preferred parameters to monitor are cell voltage or temperature, although many other parameters may be used as described above. The parameters which are measured and compared may be, for example, actual measured voltage values, or the rate of change of voltage which occurs in the sensor cell versus the at least one other cell in response to a change in operating conditions.
The output signal can have a number of effects including, for example, issuing a warning signal to alert the operator of the system to the presence of an undesirable operating condition, initiating recording of data, or triggering a corrective action to restore desirable operating conditions in the fuel cell system, by interfacing with the control system.
The monitoring of the selected parameter or parameters may, for example, be done continuously or periodically during operation of the stack.